Topic: Health & MedicineNursing

Last updated: January 7, 2020

Harare region
Programme: BSc (Hons) Nursing science
Full Name of Student: Ncube LetrishahIntake: 26 Student Pin: P995080G
Mailing Add: No. 6 Mumonda Street Mufakose, Harare
Course Name: Physiology for health sciences Course code: BSHNS 111
Assignment short Title:
Describe how the brain’s blood flow is controlled under normal circumstances.

Brain cells are very sensitive to lack of oxygen. This is because a brain cell depends mostly on oxidative metabolism of glucose for their energy source. Without oxygen they cannot produce enough energy for their upkeep. The brain therefore requires a constant supply of oxygen and glucose through the blood for its survival.

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The blood flow to the brain remains constant because there are mechanisms that prevent the blood supply from either rising too low or too high. This mechanism is inherent in the brain itself and this mechanism is said to be auto regulated.

Auto regulation of brain blood flow is depended on the amount of certain metabolic factors such as adenosine, potassium ions, hydrogen ions concentration in the brain tissue. Excess carbon dioxide and low oxygen content of brain tissue all are involved in metabolic auto regulation of brain blood flow.

When blood supply to the brain increases metabolic products like adenosine, hydrogen and potassium ions are carried away from the brain and they cause constriction of the brain blood vessels. Hydrogen ions, carbon dioxide excess and oxygen lack operate together to control brain flow in a very interesting manner.

When the oxygen supply to the brain decreases, the carbon dioxide and hydrogen ion content of the brain tissue increases. This causes the blood vessels of the brain to dilate and this is called vasodilatation. The vasodilatation increases blood supply to the brain. On the other hand when oxygen supply to the brain increases the carbon dioxide and hydrogen ion contents of the blood decreases and there is vasoconstriction of blood vessels. The main blood flow to the brain from the heart goes through the Circle of Willis where it leaves the left side of the heart through the carotid system. The vertebrae arteries join together and enter the circle of Willis. The cardiac output to the brain consists of 15% of the blood flow going to the brain.
Question 2
Describe the relationship between the ECG and the cardiac cycle.

An electrocardiogram (ECG) is a test that records the heart’s electrical activity. An ECG shows how fast the heart beats. Whether the rhythm of the heart beat is steady or irregular and the strength and timing of electrical signals are recorded as they pass through each part of the heart. The recordings are known as the electrocardiogram.

The body is a volume conductor, positively and negatively charged ions dissolved in the body fluids enable the body to conduct electricity. There are 12 standard leads, 6 limb leads and 6 chest leads. The limb leads are the bipolar whiles the chest leads are the augmented unipolar limbs. To measure the ECG using the bipolar limbs electrodes are placed on the right and left arm at the wrist and the left leg. The apices of these limbs are supposed to form an equilateral triangle with the heart in the middle of it.

Electrodes placed on the body’s surface measure the electrical potential of the heartbeat. The P, QRS and T waves represent respectively, the atrial wave of depolarization, the ventricular waves of depolarization and ventricular wave of repolarization. If a repolarization wave moves towards the recording electrode, a positive wave is recorded. During the PR interval, atrial depolarization and conduction from the SA node to AV node occur. The ventricular action potential occurs during this interval.

During the period of rest the (diastolic) the heart fills with blood. During the period of constriction (systolic) the heart contracts to eject blood.

The heart rate can be calculated from the ECG by measuring the interval between the peaks of the two successive QRS complexes. This interval is the RR interval. The reciprocal of the RR interval multiplied by 60 gives the heart rate.

Anatomy and physiology: Ross and Wilson: Waugh. A, Grant , A, Churchill Livingston: (2006): 10th edition.

Anatomy and physiology for nursing and health professionals: Golbert.B, Ankney.J, et al. Person education: (2009) 1st edition
Medical –Surgical nursing: Ignatavicius D.D, Workman L. Saunders Elsevier: United State of America: (2010) 6th edition


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